Observed climate variability and change of relevance to the biosphere

In this paper we review the current instrumental evidence regarding climate variations and change during the 20th century emphasizing those changes that are likely to have direct interactions with the biosphere. Three basic questions are addressed: (1) Is the climate getting warmer, (2) is the hydrologic cycle changing, and (3) is the climate becoming more extreme. Based on global near‐surface temperature measurements for the 20th century, it is clear that a warming of ∼0.5°C has occurred. More importantly for biospheric systems, however, are the observed asymmetric changes in daily maximum and minimum temperature, with the minimum temperatures increasing at a rate approximately twice that of the maximum temperature. Other temperature‐sensitive measures, such as glacial and snow cover extent, reinforce the observed temperature trends. Examination of the hydrologic cycle indicates that changes also appear to be occurring, although less confidence can be placed on these analyses than those for temperature. Recent studies suggest that precipitation has increased in higher latitudes, particularly in the Northern Hemisphere. Increases in cloudiness, atmospheric water vapor, and changes in stream flow also suggest that changes to a more vigorous hydrologic cycle are taking place. The final question regarding climate extremes is much more difficult to assess due to a lack of high temporal resolution climate databases. Of the few studies that have been performed, however, there is evidence that precipitation extremes, particularly heavy rainfall events, are increasing in the United States and Australia, also suggesting an enhanced hydrologic cycle as the planet warms.